Almost 1 MT/y of activated carbons are used in a very wide variety of applications. The largest volume use of activated carbons is in water treatment with other important applications including gas purification, decolourisation and adsorbency. More speciality, small volume, uses include catalysis, electrochemistry including fuel cells, biomedical devices, hydrogen storage, personal protection and automotive components. Activated carbons are used in different physical forms including powders, beads, cloths and monoliths.
Most activated carbons are made from highly abundant, low cost and essentially sustainable raw materials including coconut, coal, lignite, wood and fruitstones. The problems and limitations with such carbons include high levels of impurities, variable pore structures and limited physical forms in which they can be made stable. In particular, it would be desirable to make activated carbon-type materials with high mesoporosity for liquid phase and biomedical applications, and to make such activated carbons which can be shaped into stable forms including monoliths for good control of pressure drop and good heat and mass transfer characteristics. Such materials can be used in more specialised applications and attract higher prices.
One known approach to the preparation of more mesoporous and shaped activated carbons is through the use of synthetic organic polymer precursors. However, these are not environmentally sustainable materials being largely derived from petroleum-sourced polymers.
Another approach, which is illustrated schematically in FIG. 1A, for the preparation of mesoporous carbons is through the use of mesoporous inorganic solid templates (steps (i) and (ii)). By adsorption of a source of carbon such as sucrose into the pores (step (iii)) followed by decomposition of the carbon source (step (iv)) and dissolution of the inorganic template (step (v)), a mesoporous carbon can be formed. However, the method is multi-step, energy-intensive, and wasteful.
Another approach to the preparation of mesoporous carbons is through metal carbide precursors, for example zirconium carbide. However, these approaches require the preparation of the precursors, can be expensive and only in some cases give a mesoporous structure.
Polysaccharides are non-toxic, naturally abundant and biodegradable and as such represent a vital renewable resource for sustainable development. Like all organic materials, they can be carbonised, typically by heating to high (>300° C.) temperatures in an inert atmosphere. The materials produced in this way from ordinary native (i.e. non-modified) cellulose and starches are normally of limited value due to high microporosity and because very little control is possible in the preparation over the bulk or surface structure. There is a need to develop new, simpler and less wasteful routes to mesoporous carbonaceous and a need to design new forms of carbonaceous materials, especially with controlled bulk and surface structures, functionalisation (such as acidity) or derivatisation (such as metal adsorption) and activity in aqueous environments.
The terms “mesoporous”, “mesoporosity”, “microporous” and “microporosity are used herein in accordance with IUPAC (International Union of Pure and Applied Chemistry) standards. Mesoporosity includes pore size distributions typically between 2 to 50 nm (20 to 500 {acute over (Å)}) whereas materials with pore sizes typically smaller than 2 nm (20 {acute over (Å)}) are considered as microporous.
In the context of the present invention, “carbonisation” is used to refer to a thermal treatment process in the nature of pyrolysis in which modification of the chemical structure of the material subject to the thermal treatment process occurs, such as by modification of, or loss of, functional groups. “Carbonisation” as used herein does not require that only carbon is left as a residue after the carbonisation process, although in some cases (e.g. higher temperatures) that may be so. In other words, “carbonisation” should be understood to include partial carbonisation. “Carbonised” should be construed accordingly.
The present invention relates to new mesoporous solids comprising polysaccharide derived porous materials, or derived from polysaccharide derived porous materials by thermal treatment of the polysaccharides. In preferred materials of the invention, the polysaccharide derived porous material has a mesoporous structure and the material after the thermal treatment retains at least some of (and preferably at least a significant proportion of, and desirably most of) that mesoporous structure. The present invention also relates to functionalised and derivatised variants thereof such materials, to methods of their production and to uses thereof.
According to a first aspect of the invention there is provided a mesoporous carbonised material obtained or obtainable by thermal treatment of an expanded polysaccharide possessing acidic functionality and, optionally, one or more further expanded polysaccharides.
Preferably the acidic functionality comprises an acidic functional group which is covalently attached to the expanded polysaccharide.
In preferred embodiments, the acidic functionality comprises a carboxyl or sulfate group.
Preferably said expanded polysaccharide or, where present, one or more of said further expanded polysaccharides has a chemical structure which permits sufficient movement about the glycosidic linkage of the polysaccharide to allow formation in a fluid environment of an at least partly self-assembled mesoporous physical structure of the polysaccharide material.
In preferred embodiments, the self-assembled physical structure is a helical structure.
Preferably said expanded polysaccharide possessing acidic functionality is selected from alginic acid, pectin, carageenan and polysaccharides chemically modified to include an acidic functional group.
In preferred embodiments said expanded polysaccharide possessing acidic functionality is alginic acid.
In other preferred embodiments said expanded polysaccharide possessing acidic functionality is pectin.
In still other preferred embodiments said expanded polysaccharide possessing acidic functionality is carageenan.
Preferably the mesoporous material comprises pores in the mesoporous and microporous size distribution ranges.
Preferably the ratio of the mesoporous volume (Vmeso) to microporous volume (Vmicro) is greater than 10, when calculated using the t-plot method.
In some preferred embodiments the ratio of Vmeso to Vmicro is greater than 10 and less than 500, when calculated using the t-plot method.
In other preferred embodiments the ratio of Vmeso to Vmicro is greater than 10 and less than 200, when calculated using the t-plot method.
In still other preferred embodiments the ratio of Vmeso to Vmicro is greater than 50 and less than 500, when calculated using the t-plot method.
In yet other preferred embodiments the ratio of Vmeso to Vmicro is greater than 50 and less than 200, when calculated using the t-plot method.
Preferably Vmeso is greater than 0.2 cm3g−1.
In some preferred embodiments Vmeso is greater than 0.5 cm3g−1.
In other preferred embodiments Vmeso is greater than 0.8 cm3g−1.
In yet other preferred embodiments Vmeso is greater than 1 cm3g−1.
In still other embodiments Vmeso is less than 3 cm3g−1.
In particularly preferred embodiments Vmeso is less than 2 cm3g−1.
In some preferred embodiments the mesoporous material is partially carbonised. In other preferred embodiments the mesoporous material is substantially carbonised (that is, carbonisation of the material is substantially complete). The degree or extent of carbonisation is selectable in accordance with the desired final properties of the material.
Preferably the thermal treatment comprises heating at a temperature in the range of from room temperature to about 1200° C.
In particularly preferred embodiments the thermal treatment comprises heating from room temperature to a temperature of not more than about 700° C.
In some preferred embodiments the thermal treatment comprises heating at a temperature in the range of from room temperature to about 600° C.
In other preferred embodiments the thermal treatment comprises heating at a temperature in the range of from room temperature to about 450° C.
In still other preferred embodiments the thermal treatment comprises heating at a temperature in the range of from room temperature to about 400° C.
In yet other preferred embodiments the thermal treatment comprises heating at a temperature in the range of from room temperature to about 320° C.
In still other preferred embodiments the thermal treatment comprises heating at a temperature in the range of from room temperature to about 250° C.
In yet other preferred embodiments the thermal treatment comprises heating at a temperature in the range of from room temperature to about 200° C.
In further preferred embodiments the thermal treatment comprises heating at a temperature in the range of from room temperature to about 180° C.
In the above embodiments preferably the thermal treatment comprises heating at a temperature of not less than 100° C., more especially not less than 170° C.
In preferred embodiments the thermal treatment is carried out in a non-oxidative atmosphere, such as in vacuum or in an inert atmosphere.
Preferably the thermal treatment includes alternate heating stages and isothermal stages.
In some preferred embodiments the isothermal stages have a duration of from about 10 to about 50 minutes.
In other preferred embodiments the isothermal stages have a duration of from about 20 to about 40 minutes.
In further preferred embodiments the isothermal stages have duration of about 30 minutes.
In some preferred embodiments, in the heating stages, the rate of heating is from about 0.5K/minute to about 20K/minute.
In further preferred embodiments the rate of heating is from about 1K/minute to about 10K/minute.
According to a second aspect of the invention there is provided a method of preparing a mesoporous carbonised material comprising thermal treatment of an expanded polysaccharide, wherein the polysaccharide possesses acidic functionality and, optionally, comprises one or more further expanded polysaccharides.
Preferably the acidic functionality comprises an acidic functional group which is covalently attached to the expanded polysaccharide.
In preferred embodiments, the acidic functionality comprises a carboxyl or sulfate group.
Preferably said expanded polysaccharide or, where present, one or more of said further expanded polysaccharides has a chemical structure which permits sufficient movement about the glycosidic linkage of the polysaccharide to allow formation in a fluid environment of an at least partly self-assembled mesoporous physical structure of the polysaccharide material.
In preferred embodiments, the self-assembled physical structure is a helical structure.
Preferably said expanded polysaccharide possessing acidic functionality is selected from alginic acid, pectin, carageenan and polysaccharides chemically modified to include an acidic functional group.
In preferred embodiments said expanded polysaccharide possessing acidic functionality is alginic acid.
In other preferred embodiments said expanded polysaccharide possessing acidic functionality is pectin.
In still further preferred embodiments said expanded polysaccharide possessing acidic functionality is carageenan.
Preferably the material comprises pores in the mesoporous and microporous size distribution ranges.
Preferably the ratio of the mesoporous volume (Vmeso) to microporous volume (Vmicro) of the material obtained according to the method of this aspect is greater than 10, when calculated using the t-plot method.
In other preferred embodiments of the material obtained according to the method of this aspect the ratio of Vmeso to Vmicro is greater than 10 and less than 500, when calculated using the t-plot method.
In further preferred embodiments of the material obtained according to the method of this aspect the ratio of Vmeso to Vmicro micro is greater than 10 and less than 200, when calculated using the t-plot method.
In still other preferred embodiments of the material obtained according to the method of this aspect the ratio of Vmeso to Vmicro is greater than 50 and less than 500, when calculated using the t-plot method.
In yet other preferred embodiments of the material obtained according to the method of this aspect the ratio of Vmeso to Vmicro is greater than 50 and less than 200, when calculated using the t-plot method.
Preferably Vmeso of the material obtained according to the method of this aspect is greater than 0.2 cm3g−1.
In further preferred embodiments the material obtained according to the method of this aspect has a Vmeso of greater than 0.5 cm3g−1.
In other preferred embodiments the material obtained according to the method of this aspect has a Vmeso of greater than 0.8 cm3g−1.
In still other preferred embodiments the material obtained according to the method of this aspect has a Vmeso of greater than 1 cm3g−1.
In yet other embodiments the material obtained according to the method of this aspect has a Vmeso of less than 3 cm3g−1.
In preferred embodiments the material obtained according to the method of this aspect has a Vmeso of less than 2 cm3g−1.
Preferably the thermal treatment is such that the material obtained according to the method of this aspect is partially carbonised. Alternatively, the thermal treatment may preferably be such that the material is substantially carbonised.
Preferably the thermal treatment comprises heating at a temperature in the range of from room temperature to about 1200° C.
In some preferred embodiments the thermal treatment comprises heating from room temperature to a temperature of not more than about 700° C.
In other preferred embodiments the thermal treatment comprises heating at a temperature in the range of from room temperature to about 600° C.
In further preferred embodiments the thermal treatment comprises heating at a temperature in the range of from room temperature to about 450° C.
In yet other preferred embodiments the thermal treatment comprises heating at a temperature in the range of from room temperature to about 400° C.
In still other preferred embodiments the thermal treatment comprises heating at a temperature in the range of from room temperature to about 320° C.
In yet further preferred embodiments the thermal treatment comprises heating at a temperature in the range of from room temperature to about 250° C.
In still further preferred embodiments the thermal treatment comprises heating at a temperature in the range of from room temperature to about 200° C.
In yet other preferred embodiments the thermal treatment comprises heating at a temperature in the range of from room temperature to about 180° C.
Preferably in these embodiments the thermal treatment comprises heating at a temperature of not less than 100° C., more especially not less than 170° C.
In particularly preferred embodiments the thermal treatment is carried out in a non-oxidative atmosphere, such as in vacuum or in an inert atmosphere.
Preferably the thermal treatment includes alternate heating stages and isothermal stages.
In some preferred embodiments the isothermal stages have a duration of from about 10 to about 50 minutes.
In other preferred embodiments the isothermal stages have a duration of from about 20 to about 40 minutes.
In further preferred embodiments the isothermal stages have duration of about 30 minutes.
In some preferred embodiments, in the heating stages, the rate of heating is from about 0.5K/minute to about 20K/minute.
In further preferred embodiments the rate of heating is from about 1K/minute to about 10K/minute.
It will be appreciated that the mesoporous materials of the first aspect of the invention can be produced using the methods of the second aspect of the invention.
According to a third aspect of the invention there is provided the use of a mesoporous material according to the first aspect of the invention as a stationary phase for chromatography, as a water treatment agent, as an agent for gas purification, decolourisation or adsorbency, as a catalyst or catalytic support, as an electrode component in electrochemistry, as a hydrogen storage medium, as a filter medium, or as a component of a biomedical device.
Preferably the mesoporous material is used as a stationary phase for chromatography.
Preferably the mesoporous material is used as a water treatment agent.
Preferably the mesoporous material is used as an agent for gas purification.
Preferably the mesoporous material is used as an agent for gas decolourisation.
Preferably the mesoporous material is used as an agent for gas adsorbency.
Preferably the mesoporous material is used as a catalyst.
Preferably the mesoporous material is used as a catalytic support.
Preferably the mesoporous material is used as an electrode component in electrochemistry.
Preferably the mesoporous material is used as a hydrogen storage medium.
Preferably the mesoporous material is used as a filter medium.
Preferably the mesoporous material is used as a component of a biomedical device.
A further aspect of the invention provides a chromatography apparatus comprising a material according to the first aspect of the invention as a stationary phase.
In preferred forms, the thermally treated mesoporous materials are produced from high surface area forms of polysaccharide derived porous materials and are typically characterised by having high degrees of mesoporosity. The thermally treated mesoporous materials have surface structures which, to varying degrees, resemble those of the “parent” polysaccharide derived porous material or are more carbon-like, the relative proportion being determined by, for example, the treatment temperature (100-700° C. or more). Higher treatment temperatures will normally lead to more carbon-like structures and lower treatment temperatures will result in more retained physical and chemical structural elements of the parent polysaccharide derived porous materials. This relative proportion is usable to influence subsequent functionalisation or derivatisation of the mesoporous materials and subsequent use in different applications.
One useful functionalisation is the preparation of solid acids. To convert the thermally treated mesoporous materials of the invention to solid acids (which are useful in, for example, catalysis), the thermally treated mesoprous materials can be reacted with sulfuric acid so as to form sulfuric acid functions on the surface of the material. Alternatively, to prepare carboxylic acid functionalised mesoporous materials, a cyano group containing moiety may be grafted to the thermally treated mesoporous material and subsequently hydrolysed to give the carboxylic acid. Such solid acids can be produced in different forms such as powders and pellets, thus extending the range of applications. These materials are stable to water (including hot water); they can be used as catalysts in aqueous media, such uses including the catalysis of reactions of compounds dissolved in water. The solid acid materials are useful catalysts for the reactions of compounds in aqueous solutions obtained from the fermentation of biomass.
Suitable methods for preparing “expanded” or high surface area starches are described, for example, in WO2005/011836 and U.S. Pat. No. 5,958,589 the disclosures of which are hereby incorporated by reference. The inventors have appreciated that these described methods are, in general terms, applicable also to the preparation of high surface area polysaccharides. Polysaccharides treated by these methods are referred to herein as “polysaccharide derived porous material” or “expanded polysaccharide”. In general terms, suitable methods involve the steps of (i) thermally assisted hydration of a polysaccharide to yield a polysaccharide/water gel or colloidal suspension, (ii) allowing the polysaccharide to recrystallize and (iii) exchanging the water in the recrystallised polysaccharide with a water miscible non-solvent for the polysaccharide which has a lower surface tension than water. A suitable non-solvent is ethanol. The method can involve a series of solvents to remove the water and can involve final drying of the high surface area polysaccharide by evaporation, or can involve the use of supercritical drying including the use of liquid or supercritical carbon dioxide. The high surface area polysaccharide derived porous material can be stored as a solid material or kept as slurry in a non-solvent.
Functionalisation of the expanded polysaccharide may also, in some cases, usefully be performed, that is, prior to any thermal treatment step for carbonisation. In one particularly significant method, oxidation of the polysaccharide may be effected to introduce carboxylic acid groups. A particularly suitable method of oxidation uses H2O2 in the presence of an iron phthalocyanine catalyst. The method is generally as describe by Kachkarova-Sorokina et al in Chem Commun, 2004 2844-2845. and is summarised in FIG. 17.
An important feature of the present disclosure lies in the selection of polysaccharide materials which have mesoporous structure following the expansion process outlined above. In this way, the mesoporous structure can be retained, to a greater or lesser extent, in the materials of the invention following the thermal treatment. Without wishing to be bound by theory, the inventors believe that suitable polysaccharide materials are those which can adopt a substantially ordered structure following the thermal hydration/recrystallization process. In this process self organisation of the polysaccharides provides the desired mesoporosity. In particular, the inventors believe that polysaccharides which adopt a generally helical structure have the desired mesoporous structure. The inventors further believe that only those polysaccharides which, because of their chemical structure, permit sufficient movement about the glycosidic linkage of the polysaccharide are able to adopt the ordered structures providing the desired mesoporosity. In other words, those polysaccharides which, because of their chemical structure, have insufficient movement about the glycosidic linkage do not adopt the required ordered structure.
For example, the chemical structure of the polysaccharide may be such that movement about the glycosidic linkage is sterically limited, or there may be interactions between functional groups, such as hydrogen bonding, which limit movement about the glycosidic linkage.
The high surface area polysaccharide derived porous material can be converted directly into a carbonised mesoporous or thermally modified material of the invention by heating in suitable conditions. The heating may, in principle, be carried out at any temperature or other conditions at which suitable modification of the expanded polysaccharide, in particular partial carbonisation, substantially complete carbonisation or complete carbonisation occurs. Suitable conditions are preferably non-oxidative and desirably include vacuum conditions, or an inert atmosphere such as a nitrogen atmosphere. Conveniently, in some embodiments the heating conditions may include the use of microwave heating. In prior art methods conditions have also involved use of a catalyst (such an acid catalyst) which promotes the desired thermal modification (carbonisation). The amount of acid catalyst and its identity was varied in order to vary the subsequent processing and material properties.
Where an acid catalyst was used, one suitable method for preparing the mesoporous materials was to stir the expanded polysaccharide (prepared as described above) with 5% by weight of p-tolulene sulfonic acid in acetone, then to remove the acetone by evaporation.
In the invention, the expanded polysaccharide already contains acid functional groups (e.g. where the polysaccharide is expanded pectin or alginic acid). Thus, there may be no requirement to add an acid catalyst. In such cases, the acid-containing expanded polysaccharide is heated to about 100° C. or higher in either a vacuum or an inert atmosphere, in the absence of an acid catalyst and the desired thermally modified (carbonised) material is achieved.
The “self-carbonising” behaviour of expanded polysaccharides with innate acid functionally (.e.g. expanded alginic acid and pectin) is advantageous not only with regard to decreasing the number of process steps, but selection of such polysaccharides facilitates a reduction in the micropore content in the resulting materials. Thus, the materials derived from acidic polysaccharides display even lower micropore content than those prepared from polysaccharides which do not have acid functionality (and which normally require the use of an acid catalyst) at the same temperatures.
Generally during the thermal treatment of the expanded polysaccharide, at about 100° C. some carbonisation begins to be evident from the blackening of the polysaccharide material. The desired carbonised mesoporous materials are typically formed at temperatures of from about 100° C. to 300° C. or more, for example up to about 700° C. and possibly up to about 1200° C. Formation of (partially) carbonised materials from polysaccharide precursors at temperatures close to 100° C. is unusual and offers advantages in terms of the energy which is required and the properties that can be achieved. Carbonised materials formed using different treatment temperatures have different compositions and different properties, and in particular different surface properties. At lower temperatures (for example, less than about 300° C.) the mesoporous materials have pore structures similar to the parent expanded polysaccharide. This enables the production of carbonised materials with an unusually low degree of microporosity and an unusually high degree of mesoporosity. These materials also appear to have surface structures which are similar to, or which retain structural features (both physical and chemical) of, the parent polysaccharides (as indicated by surface energy measurements). This may be because the carbonisation process, caused by the thermal treatment, starts from the inside (core) of the polysaccharide particles and moves out. At higher temperatures (typically above about 300° C.) there is a sharp drop in the measured surface energy indicating the formation of materials which progressively bear greater similarity to a traditional carbon. These materials show an increasing amount of microporosity also typical of a more traditional carbon, although they also retain an unusual degree of mesoporosity. For example, increasing temperature may see the loss of carboxy functionality of the parent polysaccharide (at least initially, for example by conversion to carbonyl functionality), and the loss of hydroxyl functionality.
Polysaccharides that form mesoporous materials (polysaccharide derived porous materials) according to the procedure of the present disclosure are able to form ordered structures, in particular helical structures upon recrystallisation/re-association after thermally assisted hydration. These structures self assemble at the nanometer level in the presence of water, to form porous structures of a predominantly mesoporous nature. Removal of water via solvent exchange and (at least partial) carbonisation-inducing thermal treatment by the methods described herein substantially maintains this mesoporous character. The presence (if any) of microporous characteristics may be the product of the internal diameter of such polysaccharide helices, or the crystal packing of such helices and is thus polysaccharide specific. Polysaccharides that form ordered structures, such as helical structures, upon recrystallisation after thermally assisted hydration include, but are not limited to, amylose, amylopectin, pectin, alginic acid, xylan, agarose, agaropectin, xanthan gum, and modified forms of cellulose, chitin and chitosan. Mesoporous materials derived from combinations of these polysaccharides are also within the scope of the present invention, as are combinations of one or more polysaccharides which can form an ordered structure with one or more polysaccharides which do not form such ordered structures.
Without wishing to be bound by theory, the inventors believe that a one characteristic that allows the formation of mesoporous structures and, apparently, helices and/or multiple helices, subsequent to a process of thermal hydration/recrystallisation, appears to be the flexibility and degree of freedom around the glycocidic linkages of the polysaccharide. For instance, the formation of the mesoporous structures is restricted in unmodified Cellulose, (and similarly in unmodified Chitin and Chitosan), and the inventors believe that this is due to formation of very strong intramolecular hydrogen bonding, particularly between the hydroxyl group at the 3 position on the pyranose ring and the oxygen at the five position at the adjacent ring (O3HO5′) and also between the oxygen at the 6 position with the hydroxyl group at the 2 position on the adjacent pyranose ring (O6H—O2′). This promotes the formation of cellulose crystals in the presence of water, as a consequence of the adoption of a linear polymer conformation, induced by the hydrogen bonding. This prohibits the formation of the ordered structures, in particular helix formations, required for the preparation of mesoporous polysaccharides. As noted, the key characteristic that allows the formation of helices and/or multiple helices, subsequently needed to generate mesoporous materials from polysaccharides, is the flexibility and degree of freedom around the glycosidic linkage of the polymer.
It is also possible to prepare mesoporous materials according to the invention from composite precursor materials comprising mixtures of different polysaccharides. These polysaccharides can include one (or more) polysaccharide(s) that do not form ordered structures such as helices upon thermal hydration, as long as at least one polysaccharide that does form such an ordered structure upon thermal hydration is present.
The thermally treated mesoporous materials of the present invention can be functionalised or derivatised by various means. In some embodiments, the mesoporous material can be converted into a solid acid by reaction with a traditional acid or with an organic acid or precursor thereof. Suitable traditional acids include Brönsted acids, such as sulfuric acid, and carboxylic acids and Lewis acids such aluminium chloride, zinc chloride or BF3. In other embodiments, metals, such as catalytically useful metals, may be adsorbed onto or otherwise immobilised in the mesoporous material. Hetero-atoms may also be incorporated. In further alternative embodiments the mesoporous material may be prepared in combination with other starting materials, in particular polymeric materials and especially naturally occurring polymeric materials.
Users may select mesoporous materials prepared at different thermal treatment (carbonisation) temperatures for different uses since, as noted above, different carbonisation temperatures result in different surface structures, which may, in turn, react or combine (e.g. adsorb or immobilise) to different extents with functionalising agents (such as the traditional acids) or derivatising agents thereby to provide different properties in use, for example as catalysts.
In some preferred embodiments for preparing a solid acid the preferred acid is sulfuric acid, which may be used as the pure acid or as a concentrated solution in water. By stirring the carbonised mesoporous material with a concentrated solution of sulfuric acid between 20 and 100° C., sulfonic acid sites are formed on the surface of the mesoporous material. One suitable form of mesoporous material for modification with sulfuric acid is one prepared at temperatures high enough to have developed aromatic character but not high enough to have removed most of the polysaccharide-derived oxygen-containing functions, typically hydroxyl functions. Similar considerations apply to functionalisation with other Brönsted acids such as carboxylic acids.
Thus in preferred embodiments mesoporous materials prepared at about 200° C. to about 600° C. can be reacted with, for example, sulfuric acid or a carboxylic acid (or precursor thereof) to produce a solid acid that has both a good level of sulfonic or carboxylic acid functionality and also good surface polarity to assist the reactions of polar molecules, and which moreover is water-stable. This water stability makes the acid-functionalised mesoporous material suitable for catalysing the reactions of organic compounds, such as biomass-derived carboxylic acids, in water. The acid-functionalised materials prepared in this way are mesoporous and maintain a high surface area (typically greater than about 100 m2g−1).
Mesoporous materials with basic functionality may be prepared by analogous methods and are useful in, for example, base catalysed condensation reactions.
The process of preparation of the mesoporous materials can be made more specific or localised. In an example, locally functionalised or derivatised mesoporous materials can be obtained by adding suitable functionalising or derivatising moieties at the locally carbonised locations. In an example, acid species, such as sulfuric acid, are added at the locally carbonised regions so as to produce micro-channel acid-functionalised mesoporous materials for catalytic applications. In this way an array of micro-channels can be produced in a “black and white” contrast fashion, suitable for rapid screening and combinatorial-type studies.
The progress of the process of thermal carbonisation of an expanded polysaccharide material for the preparation of a mesoporous material, functionalised mesoporous material or derivatised mesoporous material can be followed by techniques including thermogravimetric analysis and by 13C magic angle spinning NMR spectroscopy. Porosimetry and surface areas can be measured, for example using automated BET measuring devices based, for example, on nitrogen adsorption. Techniques such as electron microscopy and spectroscopic probing can be used to study the surface structure, energy and chemistry.
The thermally treated mesoporous materials of the invention can be used in a number of applications where carbons, mesoporous materials and mesoporous carbons in particular find use. These applications include among others, separation, trapping, catalysis and storage. For example, the mesoporous materials can be used as stationary phases in techniques such as liquid chromatography, including high pressure liquid chromatography (HPLC). In this way, mixtures including complex mixtures can be separated on a mesoporous material using solvent mobile phases. Such mixtures may include mixtures of organic compounds, including for example pharmaceutically useful compounds and mixtures derived from natural product extracts including waxes, mixtures of organometallic compounds, and mixtures of inorganic compounds. The variable or selectable surface energy of the thermally treated mesoporous material is of particular value in separations since surface energies control the retention of eluting compounds or complexes. Thus, mixtures of compounds with different polarities can be separated using different mesoporous materials, and solvents of different polarities can be used to elute different compounds from a stationary phase comprising the mesoporous material. This includes the use of less polar solvents and the use of lower volumes of solvent than might normally be required, when using traditional silica stationary phases.
Thermally treated mesoporous materials according to the invention can advantageously be used in catalysis, including their use as catalyst supports. For example, precious metals or metal complexes can be immobilised on the thermally treated materials and the resulting supported metals can be used as catalysts in gas and liquid phase reactions including those of organic molecules. In this way, mesoporous material-based catalysts can be used in important reactions including hydrogenations, dehydrogenations, Heck and Suzuki and other carbon-carbon bond forming reactions, and oxidation. The mesoporosity of the materials of the invention makes them especially useful for liquid phase reactions such as those including reactions of large molecules. The variable or selectable surface structure of the thermally treated mesoporous materials is useful, for example to assist metal binding and to effect molecular diffusion.
More particularly, the acid functionalised mesoporous materials can be used as catalysts including the catalysis of reactions of organic compounds. Reactions can include the aqueous reactions of water-soluble organic compounds as well as other reactions under aqueous or partially aqueous conditions. This takes advantage of the stability of the thermally treated mesoporous materials towards water, including hot water, and also the presence of polar and other groups on the acid functionalised mesoporous materials, especially on those materials prepared in the temperature range of about 200° C. to about 600° C. The mesoporosity of the acid-functionalised materials also enables the diffusion of molecules, including quite large molecules, at useful rates: This diffusion does not have a large detrimental effect on the rates of reactions catalysed by these acid-functionalised mesoporous materials. Reaction types that the acid-functionalised mesoporous materials can catalyse include esterifications, isomerisations, dehydrations, cyclisations, alkylations and acylations.
Examples of reactions of importance that the functionalised mesoporous materials (in particular the acid-functionalised mesoporous materials) can catalyse include the reactions of so called biomass platform molecules. These are compounds obtained from the fermentation or other cracking process on biomass. Biomass is renewable biological material including tree materials, plants, grasses, agricultural residues and by-products and food residues and by-products. Fermentation processes on biomass normally produce aqueous mixtures of compounds. These compounds include those identified as platform molecules which can be produced in very large quantities, and which can be converted into many other useful compounds. Platform molecules of interest include succinic acid, glycerol, lactic acid, fumaric acid, levulinic acid, glutamic acid, malic acid, 3-hydroxypropionic acid, 2,5-furan dicarboxylic acid, glucaric acid, itaconic acid, sorbitol and xylitol. These molecules can be converted into other molecules including marketable products and intermediates, through the use of acid-catalysis, for example. These functionalised mesoporous materials (in particular the acid-functionalised mesoporous materials) can be used to convert compounds derived directly from biomass into numerous valuable compounds including esters, dicarboxylic acid monoesters, dicarboxylic acid esters, anhydrides, lactones, amides, dicarboxylic acid monoamides, dicarboxylic acid diamides, cyclic ethers, as well as oligomeric and polymeric substances.
The following non-limiting Examples are illustrative of the invention.